1. Field of Invention
This invention relates to optical interferometer, and particularly to optical interferometer array and miniature optical interferometer.
2. Description of Prior Art
Optical interferometer is an indispensable measurement tool in many fields. It is desirable for an interferometer to make different measurements. It is also desirable to have a small-size interferometer, which is useful for integration with other systems.
Usually, optical interference is created by splitting a beam into two parts either through amplitude division or wavefront division and then recombining the two parts. Amplitude-division may be realized by a beamsplitter and wavefront-division by a spatial phase object. An interferometer using amplitude division tends to have a larger footprint than one using wavefront division, since beams produced by amplitude division are transmitted along optical paths which are separate and head for different directions in most cases, while beams by wavefront division can have side-by-side optical paths, occupying less space.
A traditional interferometer, by amplitude division or wavefront division, is a fixed system and only good for measurement of one type. For different measurements, a tunable interferometer may be used. But a tunable interferometer is often complex and bulky. The other method involves combining a group of discrete interferometers. The interferometers each receive part of a beam respectively. This setup, however, is hardly compact.
Accordingly, there exists a need for an interferometer array, which conducts different measurements and has a compact structure.
One application of interferometer array is of spectrometer. A spectrometer is an optical instrument which measures spectral characteristics of a beam, which in turn reveals physical, chemical, or biochemical information of a sample under test. A compact spectrometer mainly contains two components: a diffraction grating and a detector array. The diffraction grating splits the beam into multiple sub-beams, where each sub-beam corresponds to a specific wavelength. The detector array is arranged such that one detector measures intensity of one sub-beam, or one wavelength. But the detector array has to be placed at a distance far enough from the grating; otherwise, neighboring sub-beams may not separate adequately, which affects resolving resolution. Therefore, current spectrometer has a limit to reduce its dimension.
In order to construct a compact interferometer array, size of individual interferometer should be small. Besides, a small interferometer as a sensor itself is convenient to integrate with a system or other sensors. For such a purpose, as discussed in the above, interferometer using wavefront division is preferred. Currently, two components are needed to build an interferometer by wavefront division: a spatial phase object for creating beam portions having different phase retardation and a focus lens for combining the beam portions to generate interference. In addition, a focus lens has a focal length, which means beam portions are combined at a distance from the lens, or focusing process takes certain space. So the present interferometer dimension is limited by two components and the beam-focusing process.
Therefore, there exists a need for a small-size interferometer which breaks the current limitation on dimension. An interferometer with a small dimension, such as subwavelength-sized or even nano-sized, may lead to other applications besides a compact or miniature interferometer array.